Zoology MCQs for NEET — Practice Questions with Answers

(d) Phosphodiester bond is the covalent bond that links a phosphate group and a sugar group by means of an oxygen bridge, in the sugar phosphate backbone of a nucleic acid molecule.

(c) Denaturation is to produce a structural change in a protein or nucleic acid that results in the reduction or loss of its biological properties. Denaturation involves unfolding of the double helix of nucleic acids, with loss of secondary and tertiary structure. It is caused by heat (thermal denaturation), chemicals and extremes of pH. The differences between raw and boiled eggs are largely a result of denaturation.

(a) Keratin is an insoluble, sulfur-containing fibrous protein that serves as the main structural molecule in hair and nails and is also present in the skin. Keratin is synthesized by cells known as keratinocytes and may be hard or soft. hard keratin is found in hair and nails, soft keratin is found in the epidermis of the skin in the form of flattened non-nucleated scales that slough continually. The main differences in various keratins arise from their sulfur content. If there are many cysteine disulfide cross-links, then there is very little flexibility as in horns, claws, hooves or nails. In wool, skin and muscle proteins, there are fewer disulfide cross-links which allows some stretching but returns to normal upon relaxation of tension.

(a) Vitamin B₆ (pyridoxine) in the form of pyridoxal phosphate coenzyme aids in the breakdown of amino acids and the metabolism of fats and carbohydrates. It also supports the central nervous system and maintains healthy skin. Deficiency of vitamin B₆ causes the epileptiform convulsions in human infants and a hypochromic microcytic anemia may develop.

(a) Succinate has a structural similarity with the inhibitor malonate. So, malonate competes with succinate (the substrate) for binding with the active site of the enzyme succinic dehydrogenase. So, it is a competitive inhibition.

(a) An example of competitive inhibition of an enzyme is the inhibition of succinic dehydrogenase by malonic acid. It is a simple type of competitive inhibition. A  competitive inhibitor resembles the substrate and binds to the active site of the enzyme. The substrate is then prevented from binding to the same active site.

(c) Lactose or milk sugar (carbohydrate) is reducing sugar formed through $\mathrm{\beta }$-1$\to$4 condensation between galactose and glucose. Lactose does not occur in nature except as a product of the mammary gland. it is highest in human milk as compared to that of cow, buffalo and goat. Lactose is a product of glucose, fructose and galactose.

(b)

(d) Enzymes are protein that, in small amounts, speed up the rate of a biological reactions and help in regulating metabolism.

Hormones arc also metabolic regulator and help in stimulation or inhibition of one or more physiological processes.

Vitamines are accessory food factors which arc required in small quantity for controlling metabolism and body functioning.

(a) The cotton fibres represent epidermal prolongations of seed coat cells. A raw cotton fibre, on average, gives the following values on analysis: cellulose-94%, protein -1.3%, pectic subtances-0.9%, ash-1.2%, wax-0.6%, sugars-0.3% and traces of pigments and mineral matter.